External System as Actor

In this use case diagram example, actors include the clerk, supervisor, and Ticket Vending Machine. The Ticket Vending Machine is another system that accepts orders from a customer. The customer is not an actor in the box office application because the customer is not directly connected to the application. Use cases include buying tickets through the Ticket Vending Machine or the clerk, buying subscriptions (only through the clerk), and surveying total sales (at the request of the supervisor). Buying tickets and buying subscriptions include a common fragment—that is, making charges to the credit card service. (A complete description of a box office system would involve a number of other use cases, such as exchanging tickets and checking availability.)

Use cases can also be described at various levels of detail. They can be factored and described in terms of other, simpler use cases. A use case is implemented as a collaboration in the interaction view.